Aberrant regulation of cholesterol and lipid homeostasis leads to metabolic syndrome and cardiovascular diseases. microRNAs (miRNA) are short non-coding RNAs that control gene expression predominantly through post-transcriptional repression. They are implicated in the control of multiple physiological and pathological processes. However the specific roles of miRNAs in regulating cholesterol and lipoprotein metabolism are just beginning to be explored. Our previous work has shown that hsa-miR- 33a/b and its mouse homologue mmu-miR-33 (herein after referred to as miR-33), intronic miRNAs located within the sterol-regulatory element-binding protein (SREBP) 2 and 1 genes, respectively, in humans; regulate cholesterol homeostasis in concert with the SREBP host gene. MiR-33 inhibits the expression of the ATP-binding cassette (ABC) transporter, ABCA1, thereby attenuating cholesterol efflux to apoA1 and high-density lipoprotein (HDL) biogenesis. Conversely, silencing of miR-33 in vivo increased hepatic ABCA1 and plasma HDL. In addition, our recent preliminary data suggest that miR-33 also coordinates genes regulating fatty acid metabolism and insulin signaling. For example, miR-33 targets carnitine O- octaniltransferase (CROT), Carnitine palmitoyltransferase 1A (CPT1a) and hydroxyacyl-CoA- dehydrogenase (HADHB), key enzymes involved in the regulation of fatty acid oxidation, and insulin receptor substrate 2 (IRS2), which regulates insulin signaling. Thus, we hypothesize that inhibition of miR-33 may represent a therapeutic target for ameliorating cardiometabolic disease, including atherosclerosis and metabolic syndrome. The objective of this proposal is to determine the molecular mechanism underlying the miR-33-mediated responses in cardiometabolic disease. We propose the following specific Aims: Aim 1. To delineate the role of miR-33 in regulating cholesterol metabolism, - oxidation of fatty acid and insulin signaling in vivo and Aim 2. To define the role of miR-33 in lipid metabolism, insulin signaling, and atherosclerosis in vivo.